Minimum detectable signal

Minimum Detectable Signal (MDS) - in a radio receiver, the smallest signal power that can be received at its input, processed by its conversion chain and demodulated by the receiver resulting in useable information at the demodulator output.

MDS = -174dBm + 10log(bandwidth, Hz) +SNR (dB)

In this equation:

-174dBm represents the noise power due to KTB watts where B = bandwidth in Hz, T is the system temperature and K is Boltzmann's constant. If the system temperature and bandwidth is 290K and 1Hz, then the noise power available in that 1Hz bandwidth is -174dBm (174dB down on one milliwatt). This is the absolute noise floor for a system.

If the bandwidth of the information signal is not 1Hz, then the term 10log(bandwidth) allows for the additional noise power present in the wider detection bandwidth.

SNR (singal-to-noise ratio) is the degree to which the final demodulated signal needs to be greaster than the noise floor in order for the demodulator to generate the useful ionformation at its output. In the case of digital systems a 10dB difference betweeen the noise floor and the signal level is needed; this 10dB SNR allows a bit error rate (BER) to be better than some specified figure (e.g. 10^-5 for some QPSK scheme). For voice signals the required SNR might be as low as 6dB and for CW (Morse) it might extend, with a trained listener, down to 0 dB difference.